SOMETIME AROUND NOON I started to appreciate the math: The fact that I was riding my bicycle to my 20-year high-school reunion meant that I wasn't quite as young as I'd been picturing myself all these years. I was creeping up a hill, badly out of breath, but the origin of this revelation was neither my legs nor my lungs. It was the two spots on my buttocks that made me wonder if the bicycle's seat was made of salt and razor blades: saddle sores.

I was not sure exactly where the idea of riding 200 miles along the Mississippi River, from my current home in Minneapolis down to my childhood home of Winona, came from. Maybe it was some kind of midlife bid to escape the icy grip of domesticity--nearing 40, I had a wife, two kids and a house. Maybe deep down I still wanted to prove something to my old classmates. All I knew for sure was that once the idea grabbed hold of my psyche it wouldn't let go, and now, just a few hours into the trip here I was, out on the road, loaded down with gear and memories and doubt.

"Hola," I said to the man walking past me. I was not far south of Minneapolis, but I was already on a part of the river I had no idea existed.

"Mucho pescado," the man said, pointing over to where his friends were fishing. He was from Mexico. He had cowboy boots and a belly that pushed out his giant belt buckle. He looked at my loaded bike and sweat-drenched shirt.

Winona was beautiful. That was what everyone said about it, and about the valley that held the town. And it's true. The road that passes through it and winds up the Mississippi River has long been considered one of the most scenic drives in the country. Likewise, that segment of the 3,000-mile Mississippi River Trail for bicycles is one of the most popular.

But growing up there, I could never see the beauty. In spite of the town's pretty panoramas, there was something dark and gnarled at its core. For the whole year of seventh grade, I had to avoid my locker because it was in the basement where the tough kids hung out. I still remember the day in junior high school when our principal got punched in the face with a spiked glove, and another day when the vice principal got the same, giving him a long row of stitches under his eye. There was practically regular bus service heading up the river to a reform school, from which no one came back reformed. We convened on dead-end roads in the Mississippi backwaters, hundreds of us standing around, holding plastic cups full of watery beer. The cops would block us in and we would run though the woods, or swim if we had to.

In four out of the five categories of violent crime tracked nationally, and in five of the five property-crime categories, Winona ranks higher than the U.S. average. Not long ago, when I saw a headline about a 12-year-old getting a DWI, I knew before I read the story that it had happened in Winona. And when I read about a murderer serving on a jury in a murder trial (there were no objections), I was not at all surprised.

In time, and with distance, I came to think that the wildness must be something in the town's cultural DNA. Winona had a long history of being a chaotic river town, one in which, for example, the red-light district operated openly until the governor sent in special operatives to shut it down in the 1940s.

"You bike here?" a voice shouted as I walked from my tent to the shower that night.

"Yeah," I answered. "From Minneapolis."

"Good Glory!" He sounded alarmed. "I'd be lucky if I could bike across this parking lot."

It had taken me all day, but I had finally arrived at the Prairie Island Indian Reservation, where I'd set up camp in the shadows of both a huge casino and an even bigger nuclear plant. But I was right next to the river, on a piece of ground dotted with burial mounds.

The man turned to a shiny Mustang, got in, and began backing it under a metal awning.

"Nice car," I said.

"I want to keep it that way. Storm coming. Said they had golf-ball-size hail in North Dakota."

I looked up to see the sky turning an unsettling shade of brownish orange.

After dark, I climbed into my tent, and the wind picked up. I brought my helmet inside, just in case of hail. The wind started to roar, a low guttural sound, and I felt small and exposed and vulnerable. Out my tiny plastic window I could see the trees pitching back and forth. Then came the rain in sheets. Tornado sirens began wailing across the river.

I jumped out of my tent and ran for the cinder-block bathroom, where I huddled for an hour, peeking out the door and expecting it to be ripped off the hinges.

When I woke up the next day, the air was cool and clear. The sun was bright, and there wasn't a cloud in the sky. I packed my gear and got back on the road. My tires felt fast, and I felt alive in a way I hadn't for some time.

Just off the reservation, I hit my first big hill, around half a mile long and at least 500 feet high. I could feel the climb in every inch of my body from my waist down. But flying down the other side, free from exertion, I remembered when I'd discovered, just before leaving home, what excellent cycling country I'd grown up in. I'd begun taking my bicycle up into the bluffs, riding and riding. There were long climbs, not many cars, uncountable rolling hills. Those were the same roads on which Greg LeMond was riding to train for the Tour de France. I didn't know that back then. But I didn't know a lot of things, including how far two pedal-powered wheels could take me.

Later, miles from the reservation, I came to a place called Barn's Bluff, a giant limestone mound that sticks straight up out of the Mississippi River Valley floor. Almost exactly 150 years ago, Henry David Thoreau had stopped here on his way back down the river. His Walden days were long behind him. He was just a year away from dying, and he spent most of his time poking around in the weeds, collecting plant samples. Even so, I wanted to see, more or less, what he had seen. So I stashed my bike behind a small building and started climbing.

The trail wound up around the back of the hill, through woods, then finally up an old concrete staircase, where it emerged into a meadow the length of the hill. Below, the valley opened in its giant yawn.

It was green and blue as far as I could see. The river fanned out for a mile before rising up on the other side. Sun shone off the water. Some birds with wide wingspans drifted overhead.

Much here was different from when Thoreau sat writing letters. Yet, the hill was the same. The valley was the same. And the river was still massive, still flowing to the sea. I had one of those brief moments when a place we know so well suddenly seems like an exotic, beautiful, lost world we are seeing for the first time. Maybe that's what the mysterious point of my trip home was all about.

I got on my bike and rode over mile-long hills, through hidden valleys, past the shell of a once-luxurious hotel where Ulysses Grant and Millard Fillmore and William Randolph Hearst had stayed. I almost rode over a snake sunning itself on the road. I came to a place called Wabasha, where I found myself at the National Eagle Center, a giant gift shop masquerading as an educational facility about birds.

That was where I ran into a kid with a bright yellow Bike Across America shirt, and stopped to talk to him. It turned out he was in the crew, not riding. The cyclists, he said, are "mostly retired people," headed from west to east.

They weren't alone. The roads, it seemed, were full of people biking across America. But almost none were going the way I was, north to south. I'd seen only a few other cyclists ride the Mississippi River Trail for any length of time.

That night, when I got to my campground, I asked at the desk if they got many cyclists riding the river.

"Yeah, we do!" the man checking me in said.

"Mostly from Europe, though. I think we're on some map that says this a good place for bikers."

The next day, I continued south, across a large plain I'd never seen, to a town I'd never heard of, where I sat and watched a deer slip into the river and swim across a channel. I rode through some ancient sand dunes that must have resembled what this area was before it turned into farmland. I watched a mink cross the road in front of me. I was having such a good time exploring, discovering just how little I knew about the valley where I grew up, that I almost forgot where I was going--until a small voice reminded me.

"Get out of here, you ugly weirdo!" a kid yelled from his porch. He stood next to his parents, who looked at me coldly. I waved, then sped away as a grossly overweight basset hound ran out and tried to bite my ankle off.

I was very close to home now.

For 20 years, I'd been trying to be not from here. I hated my hometown's inwardness, its death grip on the things that mattered least, and for two decades I'd tried to expunge the part of me born here and replace it with pieces from everywhere else.

I turned off the main highway and pedaled up the road for a mile or so, until I came to a hastily torn piece of cardboard with "1990" scribbled on it. The sign was propped up against a pole. An arrow pointed left. I turned. Just then, a car full of girls--I mean, middle-aged women--drove past me then slowed down, and I heard my name being yelled. I waved and rode on through a rush of strangely warm feelings.

It was starting to get dark when I rolled across the lawn filled with cars and people standing around. I parked my bike and found the host, who gave me a big, friendly handshake. Then I went inside to change my shirt and came out to reunite.

"Frank!" someone said. "You surviving?"

I said that I was surviving. He looked familiar, but I couldn't find a name. I looked around.

"Frank! Did you bike here? I almost hit you on the way down!" said an old friend named Rod. Other people overheard and joined in.

"You drove your bike down here?"

"Hey, did you hear he rode his bicycle all the way down here?

"Holy shit! What, you don't own a car?"

My head swam in a sea of distorted faces. It was a high-school reunion, with all the usual surprises. The headbanger had turned into the salesman of the year. The quiet girl was a karaoke queen. The guy who used to run through the woods from the cops was now a cop chasing kids through the woods. We couldn't believe we were so old, that it had been so long.

A woman with big hair interrupted one of my conversations to say, "You guys might want to move." She pointed to one of our classmates, who was lighting a bonfire with a propane tank.

"It's okay," another woman said. "He's a fireman."

The night went on. The volume rose. Reminiscences and laughter floated on an ocean of watery beer. Eventually, perhaps inevitably, there was a fight--two women pulling hair and giving black eyes.

By the time the cops came, I was long gone, riding down a deserted road next to the Mississippi River, past the fires of people camping by the shore. As I rolled through my hometown, it was dark, but it was beautiful, too, and everything was quiet except for the hum of my wheels on the pavement and, far off, the hush of the water closing behind me.